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The Climate Impacts Group (CIG) translates global-scale climate forecasts
and conditions into regional-scale climate forecasts for Pacific
Northwest (PNW) resource managers and the general public. The
El Niño/Southern Oscillation (ENSO) is
the most important factor for seasonal forecasting, changing the odds
for different types of winter and spring weather (e.g. warmer/drier, cooler/wetter)
in the PNW. Another important climate variable for Pacific Northwest climate is the Pacific Decadal Oscillation (PDO). The climate outlook also provides the basis for natural resource
forecasts, including the CIG's annual streamflow forecasts.

What's Next for the Pacific Northwest?

October 2010
Updated 21 October 2010 (posted 25 October)

The climate outlook is reviewed monthly and updated as needed.

The July-August-September mean sea surface temperature anomaly in the
Niño 3.4 region is -1.25°C, which is consistent
with moderate "La Niña" [cold El Niño / Southern
Oscillation (ENSO)] conditions. The
consideration of current conditions and model forecasts has led
NOAA to expect that cold ENSO
conditions will "last at least into
Northern Hemisphere spring 2011" (7 October, NOAA).
A discussion of current equatorial
Pacific conditions and model forecasts is provided below.

The NOAA Climate Prediction Center forecast for November-December-January (NDJ) precipitation is for a greater than 33
percent chance of above normal precipitation throughout the
Pacific Northwest, western Montana and Wyoming, and the northern
portions of Utah, Nevada, and California. Chances of above normal
precipitation exceed 40% in north and central Idaho, western and
northern Oregon, and all of Washington.

The NDJ temperature forecast is
for a greater than 33% chance of below normal temperatures in
northern, central, and western Washington, western
Oregon, and in the coastal region of northern California.
The changes of the same exceeding 40% in the coastal areas of
Washington and Oregon. Idaho, southeast Washington, and
eastern Oregon are forecast to have an equal chance of below, near, and above normal
temperatures.

The existence and forecast persistence of a strong ENSO episode
makes it useful to also discuss the climate forecast for later in the
winter. The FMA precipitation forecast is qualitatively
similar to the NDJ forecast. The FMA temperature forecast is highlighted by a greater than 50%
chance of below normal temperatures for North Dakota, northern
Montana and Idaho, all of Washington, and northern and western
Oregon.

The seasonal forecasts should be interpreted as the tilting of odds towards
general categories of conditions, and should not be viewed as
a guarantee that the specified conditions will be
realized.

For More Information

The western U.S. experienced above normal temperatures during the
30-days ending 19 October (1971-2000 mean, WRCC). The largest temperature
departures, in excess of 6°F (3°C), were observed in norhtwest Nevada, southeastern Oregon, central and
southern Idaho, southern Montana, and Wyoming. For the Pacific
Northwest, this represented a distinct shift from the cooler than
normal temperatures that have dominated the region since spring (6-month
average temperature; Climate
Outlook archive).

The 30-day accumulated precipitation ending 19 October saw below
average precipitation in western Oregon and southern Idaho, and above
normal precipitation along the northern Washington coast (precipitation totals, departures, percent of 1971-2000 normal).

The cool, wet conditions throughout the growing season has led to
Oregon and Washington grape harvests that are from 10-20 days later
than usual. A consequence of the late harvest is that grape losses
from migrating birds could be substantial (23 September, Tri-City Herald
(Kennewick, Pasco, and Richland); 21 October, Oregonlive).

The Northwest Interagency Coordination Center (NWCC) maps the locations of
"large wildland fires" (timber > 100
acres, grassland > 300 acres; 100 acres = 0.16 square miles) for
Oregon and Washington. The fire season is drawing to a close
and, on 21 October, there remain 5 large wildland fires in Idaho,
2 in Oregon, and none in Washington (Inciweb). The 19 October Drought Monitor identifies an
increase in drought area and severity in south central Oregon and
the onset of drought in southern Idaho. The enhanced drought
conditions are consistent with the drier, warmer than normal
climate in the drought regions during the last 30 days.

September coastal
sea surface temperatures (SSTs) continued the pattern of below normal
temperatures that have been observed since April of this year (PFEL). SST
departures in excess of 1 °C colder than a 1985-97
mean were observed from Tatoosh Island at the northwest
corner of Washington to Astoria in northern Oregon, and also to the south of Cape Blanco
(42.83°) in
southern Oregon.

On a broader scale and, with a cooler reference
mean, SST departures for the northeast Pacific SST for the 30 days
ending 16 October were warmer than a 1982-96 normal near the Washington
and Oregon coast, near normal farther offshore, and cooler than the
mean in the central Gulf of Alaska (centered at 45°N,145°W).
Significant negative SST departures are present in the central and
eastern Pacific, and to the south and east of Hawaii, and are associated with
the ongoing cold ENSO episode.

El
Niño/Southern Oscillation
(ENSO).
Sea surface temperature (SST) departures from 1971-2000 averaged over 5°N-5°S, 170-120°W in the equatorial Pacific, in what is called
the "Niño 3.4" region, are a key indicator
of ENSO variability. The ongoing cold ENSO, as measured by Nino 3.4 SST
departures, intensified from August (-1.18 °C)
to September (-1.56°C). The mean Niño
3.4 SST
departure for July-August-September (JAS) was
-1.25°C, which NOAA categorizes as a
moderate "La Niña" when accompanied by
consistent atmospheric anomalies (NOAA El
Niño Southern Oscillation (ENSO) diagnostic
discussion).

NOAA
employs mechanistic and statistical models to forecast how ENSO will
evolve over the next several seasons. Mechanistic models solve
equations for ocean and atmosphere motions, and precipitation and radiative processes to
forecast the future from present conditions.
Statistical models, in contrast, are
constructed from observations of past climate, and
they apply regression coefficients to present climate
conditions to forecast the future. Agreement of the forecasts from these two types of
models increases our confidence in the forecast.

The 3-month mean Nino 3.4 SST forecast from twenty-three ENSO models initialized with ocean and atmosphere data
through September are summarized by
the International Research Institute for
Climate and Society. The average forecast is for the cold ENSO to
intensify from the observed JAS value of -1.25°C
to a
maximum negative value of -1.8°C in
October-November-December. A 3-month Niño
3.4 SST anomaly value in excess of
-1.5°C, would make this a "strong" La
Niña (cold ENSO). The forecast for subsequent seasons
is for the episode to diminish in intensity, with
the "La Niña" rating to be that of "moderate" (Niño
3.4 > -1.5°C) in
January-February-March, "weak" (Niño 3.4 >
-1.0°C) in March-April-May,
and a return of conditions to "neutral"
(Niño 3.4 > -0.5°C) by next
summer. There is considerable range in the
forecasts Niño amplitudes, but good
agreement on the trajectory of the episode.

Pacific Decadal
Oscillation (PDO).
The continued intensification of positive SST departures in the central
north Pacific during September contributed to a
further intensification of negative PDO conditions
during this month, resulting in a PDO value of -1.61 standard
deviations (1900-90
reference period, PDO
values). [For a normally distributed variable, only
32% of the values exceed one standard deviation in
magnitude.] September is the fourth consecutive month
of negative PDO values following positive PDO values that were observed last winter and
spring. ENSO variabilty is one
of the mechanisms that can produce changes in the
PDO, and the present cold ENSO can be expected to
contribute to negative PDO values in the coming seasons.

NOAA employs both
statistical and mechanistic models to forecast the
PDO and coastal ocean conditions. The statistical
linear
inverse model predicts the PDO to be negative and strengthening through
the end of the forecast period in summer 2011.
The NCEP coupled forecast system (a mechanistic
ocean-atmosphere model) prediction issued on 21
October is
for a continuation of
the present pattern of cold SST anomalies along
the eastern boundary north of 40°N (Cape
Mendocino in California) and
warm anomalies in the central north Pacific
through March-April-May 2011 (21 October forecast, more recent
forecasts). The model forecasts for
next winter and spring have
been judged to not be skillful for the central and
southern California coast.